1. Technical Field
This disclosure relates to an image forming apparatus, and more specifically to an image forming apparatus including a recording head for ejecting liquid droplets.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, an inkjet recording apparatus is known that uses a recording head (liquid-droplet ejection head) for ejecting droplets of ink. Such inkjet-type image forming apparatuses fall into two main types: a serial-type image forming apparatus that forms an image by ejecting droplets from the recording head while moving a carriage mounting the recording head in a main scanning direction, and a line-head-type image forming apparatus that forms an image by ejecting droplets from a linear-shaped recording head held stationary in the image forming apparatus.
As for the recording heads used in these liquid-ejection-type image forming apparatuses, several different types are known. One example is a piezoelectric recording head that ejects droplets by deforming a diaphragm using, e.g., piezoelectric actuators. When the piezoelectric actuators deform the diaphragm, the volumes of chambers containing the liquid change. As a result, the internal pressures of the chambers increase, thus ejecting droplets from the head. Another example is a thermal recording head that ejects droplets by increasing the internal pressures of chambers using, e.g., heaters disposed in the chambers. The heaters are heated by electric current to generate bubbles in the chambers. As a result, the internal pressures of the chambers increase, thus ejecting droplets from the head.
For such liquid-ejection type image forming apparatuses, there is demand for enhancing throughput, i.e., speed of image formation. One way to increase the throughput is to enhance the efficiency of liquid supply. For example, a tube supply method is proposed in which ink is supplied from a large-volume ink cartridge (main tank) mounted in the image forming apparatus to a head tank (also referred to as a sub tank or buffer tank) mounted in an upper portion of the recording head through a tube.
In this regard, in a case where ink is supplied from the ink cartridge to the head tank via a tube made of, e.g., resin, it is difficult to use the head tank with the head tank constantly full of ink and an air layer is formed in an upper space of the head tank. The amount of air in the head tank is likely to increase over time due to air permeating from wall faces of the resin tube and the head tank or air bubbles entering the tube at the installation and removal of the ink cartridge.
A small amount of air in the head tank is not so problematic. However, if the amount of air in the head tank is too large, the amount of change in the volume of air relative to temperature change increases. As a result, the internal pressure of the head tank may be out of a proper range of negative pressures to be maintained, thus leaking ink from nozzles of the recording head or hampering normal ink ejection. In addition, when the amount of air in the head tank is too large, air may mix into ink, thus hampering normal droplet ejection.
Therefore, it is preferable to control the amount of air in the head tank below a threshold amount while maintaining the internal pressure of the head tank within a proper range.
Hence, for example, JP-2010-120263-A proposes a liquid ejection apparatus that has an exhaust mechanism including an air storage part, a valve, and a flexible member. The air storage part is disposed at a liquid supply channel for supplying liquid to the recording head and temporarily stores air contained in ink. The valve opens and closes an exhaust passage leading from the air storage part to the outside. The flexible member is deformed by negative pressure generated in the exhaust passage to open the valve and exhaust air from the air storage part to the outside through the exhaust passage.
However, in the above-described configuration, by negative pressure (exhaust pressure) generated in the exhaust passage, the flexible member is deformed to open the valve. As a result, when the valve is opened, the exhaust pressure may affect the internal pressure of the head, thus sucking air from the nozzles into the liquid ejection head.
In other words, a large negative pressure need be applied to the exhaust passage to open the valve, and once the valve is opened, the large negative pressure directly acts on the liquid ejection head. In particular, in a case where air is exhausted from a plurality of air storage parts, a larger negative pressure need be applied to the exhaust passage, thus making it difficult to perform exhaust operation with the pressure of the head stably maintained.